Agricultural spraying by economical and available technologies uses hydraulic spray nozzles that inherently produce a wide spectrum of spray droplet sizes. The potential for these spray droplets to drift from the initial, desired site of application is found to be a function of droplet size, with smaller droplets having a higher propensity for off-target movement. Significant research efforts, involving numerous field trials, wind tunnel tests and subsequent generation of predictive math models have led to a greatly enhanced understanding of the relationship between spray droplet size and potential for off-target drift. Although other factors such as meteorological conditions and spray boom height contribute to the potential for drift, spray droplet size distribution has been found to be a predominant factor. Teske et. al. (Teske M. E., Hewitt A. J., Valcore, D. L. 2004. The Role of Small Droplets in Classifying Drop Size Distributions ILASS Americas 17th Annual Conference: Arlington Va.) have reported a value of <156 microns (μ) as the fraction of the spray droplet distribution that contributes to drift. Wolf (www.bae.ksu.edu/faculty/wolf/drift.htm) cites a value of <200μ as the driftable fraction. A good estimation of droplet size likely to contribute to drift, therefore, is the fraction below about 150μ.
The negative consequences of off-target movement can be quite pronounced. Some herbicides have demonstrated very sensitive phytotoxicity to particular plant species at extremely low parts per million (ppm) or even parts per billion (ppb) levels, resulting in restricted applications around sensitive crops, orchards and residential plantings. For example, the California Dept of Pesticide Regulation imposes buffers of ½-2 miles for propanil containing herbicides applied aerially in the San Joaquin valley.
High molecular weight, water-soluble polymers are sometimes added to spray compositions as a tank mix to increase droplet size and thereby reduce drift (see, for example, WO 2008/101818 A2 and U.S. Pat. No. 6,214,771 B1). However, high molecular weight, water-soluble polymers are not entirely satisfactory because they do not always work with many aerially applied herbicide tank mixtures, due to pump shear, wind shear and other performance issues, which are more pronounced in high speed aerial application conditions. See Hewitt, A. J. (2003) Drift Control Adjuvants in Spray Applications: Performance and Regulatory Aspects. Proc. Third Latin American Symposium on Agricultural Adjuvants, Sao Paolo, Brazil. 